Metal solution-diffusion membrane and method for producing the same

ABSTRACT

A metal solution-diffusion membrane of a macroporous base on which a thin metal membrane layer is formed is disclosed. In the membrane, the base includes a hollow fiber with a metal material containing an intermediate layer being formed between the hollow fiber and the metal membrane layer. The metal solution-diffusion membrane can be made with a very thin metal membrane layer with high permeability and possesses long-term stability as well as a great separation surface/volume ratio. A method for producing a metal solution-diffusion membrane in which the intermediate layer serves to provide nuclei for the subsequent currentless deposition of the metal layer is also disclosed.

FIELD OF THE INVENTION

The present invention relates to a metal solution-diffusion membranefrom a macroporous base on which a thin metal membrane layer is formedas well as to a method for producing this metal solution-diffusionmembrane.

Metal solution-diffusion membranes play a major role in purifying orfiltering gases in industrial processes. The growing demand for hydrogenas a fuel or as a reaction product in the chemical industry hasattracted the attention of research to the production, purification anduse of hydrogen. Purifying respectively filtering hydrogen plays a vitalrole in this. Particularly metal solution-diffusion membranes, as forexample palladium membranes, are especially suited for separating andpurifying hydrogen for applications in the electronics, the metal or thechemical industries. The drawbacks associated with palladium membranesare, in particular, little permeability and little long-term stabilityas well as a low separation-volume ratio.

DESCRIPTION OF THE PRIOR ART

Hitherto palladium membranes were usually applied onto large surfaceporous carrier bodies. For example, it is prior art to produce palladiummembranes in the form of foils that are then applied onto a carrierstructure. Foils of this type, however, can only be produced with aminimal thickness of usually approximately 7 μm so that permeability isnot high enough for some applications.

A method of producing a metal-ceramic catalyst membrane for producing avery thin membrane layer on a macroporous base is known from H. Zhao etal's, Catal. Today, 1995, 25, 237 to 240. In this method, a thinmetal-containing membrane layer is generated on a macroporous ceramicbase. The metal is applied onto solid state particles, which aregenerated as a thin covering layer on the carrier body.

A disadvantage of this thin membrane layer as well as of theabove-mentioned foil membranes, however, continues to be insufficientlong-term stability. In certain applications, such type membranes areexposed to high temperatures so that the difference in thermal expansioncoefficients between the base and the metal membrane layer inconjunction with the turning brittle of the metal layer upon contactwith the hydrogen leads to great stress, which can lead to detachment ofthe base and the membrane layer at the not optimum joints. Especiallywith the usually employed large-surface plate-shaped base bodies, thiscan lead to function failure of the membrane.

The object of the present invention is to provide a metalsolution-diffusion membrane as well as a method for producing the same,which possesses higher long-term stability with high surface-volumeratio and greater permeability.

SUMMARY OF THE INVENTION

The object of the present invention is solved with the metalsolution-diffusion membrane and the method according to the claims.Advantageous embodiments of the membrane and the method are the subjectmatter of the subclaims.

The present metal solution-diffusion membrane comprises a macroporoushollow fiber as the base on which a thin metal membrane layer is formedover at least one thin, metal material-containing intermediate layer.

The combination of a hollow fiber as the base with a thin metal coatingyields a completely encompassing metal membrane layer which does notlose its filter property even due to detachments of the base at notoptimum joints. Such type local detachment therefore does not lead tofunction failure of the membrane.

Optimum function of the thin membrane layer requires a uniform,homogeneous substructure which, in the present membrane, is formed as anintermediate layer between the hollow fiber and the metal membranelayer. Especially this intermediate layer permits realizing a very thinmetal membrane layer on the hollow fiber. The very thin membrane layer,for its part, leads to very high permeability of the membrane, forexample for hydrogen. Furthermore, the use of a hollow fiber as the baseyields a very good surface/volume ratio. A multiplicity of such typecoated hollow fibers can be used in a filter element.

The metal membrane layer can, for example, form a layer thickness in therange between 0.1 and 10 μm. Preferably, it has a layer thickness in therange between 0.7 and 1 μm. The intermediate layer can possess a layerthickness between 1 and 10 μm, preferably the layer thickness of theintermediate layer is between 2 to 3 μm. In the preferred embodiment,this intermediate layer is formed of particles of a sol, which arecoated with a salt of the metal of the metal membrane layer. The poresize of the intermediate layer lies preferably in the range ofapproximately 6 nm. The production of such a type intermediate layer is,for example described in J. Zhao et al's above mentioned publication.The layer produced as a membrane layer therein acts in the presentmembrane as an intermediate layer.

The employed hollow fibers preferably have an outer diameter in therange between 80 and 1500 μm, a wall thickness in the range between 10and 200 μm as well as an average pore size of about 0.2 μm. A smallerouter diameter is associated with less wall thickness.

The hollow fibers can, for example, be formed from a ceramic or ametallic material. For this purpose, Al₂0₃ is especially suited as theceramic material. Metals for the metal membrane-layer are preferablypalladium, nickel, platinum or alloys thereof. Other metals can also beemployed for gas separation processes. Examples therefor are copper,iron, silver, aluminum or alloys thereof.

In the present method for producing the metal solution-diffusionmembrane, one or a multiplicity of macroporous hollow fibers areprovided or produced on whose surface a homogeneous intermediate layeris applied which contains metal nuclei for subsequent currentlessdeposition of a metal membrane layer. The intermediate layer is thenpassivated. Finally, the thin metal membrane layer is applied onto thisintermediate layer by means of currentless deposition.

Preferably the application of the homogenous intermediate layer occursusing a method according to J. Zhao et al's above mentioned publication,i.e. application of a Böhmit sol modified with metal complexes andfollowed by calcination.

In a preferred embodiment in which the pore size of the macroporoushollow bodies for direct application of the sol is too large, anadditional intermediate layer with a smaller pore size is applied ontothe hollow fiber, so that the sol particles cannot or only to a smallextent penetrate into the pores.

A BRIEF DESCRIPTION OF THE DRAWINGS

The present metal solution-diffusion membrane and the method to producethe same are made more apparent in the following using a preferredembodiment with reference to the drawings without the intention oflimiting the scope or spirit of the overall inventive idea.

FIG. 1 shows the basic buildup of the present solution-diffusionmembrane and

FIG. 2 shows a schematic representation of the layer structure of thepresent solution-diffusion membrane.

WAYS TO CARRY OUT THE INVENTION

FIG. 1 shows in a very schematic representation a cross section of thebasic buildup of the present solution-diffusion membrane using apreferred embodiment, in which a single intermediate layer 2 is disposedbetween the metal membrane layer 3 and the hollow fiber 1. Thisintermediate layer 2 provides, on the one hand, the nuclei forcurrentless deposition of the metal layer 3 and acts, on the other hand,as a homogeneous substrate for the currentless deposition of the metalmembrane layer 3.

In the present example, a macroporous α-Al₂O₃ hollow fiber which, forexample, can be produced using a spin-extrusion technique is provided asthe hollow fiber. The hollow fibers utilized in this preferredembodiment have an outer diameter of 700 to 800 μm, an inner diameter of500 to 600 μm and an average pore size of 0.2 μm. A Böhmit sol modifiedwith a palladium complex is applied onto the surface of these fibers inorder to place palladium nuclei on the surface of the ceramic hollowfibers. These palladium nuclei act as catalysts for the subsequentcurrentless deposition of palladium.

The modified sol is applied using a dip coating process. Simultaneouslya vacuum is applied to the inner volume respectively the hollow channelof the hollow fibers. The particle size of the sol is approximately 60to 100 nm. By applying a vacuum to the inner surface of the hollowfibers, a small part of the sol particle penetrates into the pores ofthe hollow fibers thereby improving adhesion of the intermediate layerto the hollow fiber.

Following this, calcination occurs at 750° in the air. Then the surfaceis reduced respectively passivated at approximately 200° flowinghydrogen.

The average pore size of the yielded Pd/γ-Al₂O₃ intermediate layer isabout 5.7 nm in this execution of the method. The narrow pore sizedistribution and the very homogeneous surface of this intermediate layerpermits the subsequent defectless deposition of an ultra thin palladaiumlayer. The currentless deposition process is based on a chemicalreaction between [PdEDTA]²⁻and hydrazine under the catalytic effect ofthe palladium nuclei. In this method, the surface of the hollow fibersrespectively the intermediate layer applied thereupon is covered with acontinuous, dense palladium layer which has a little thickness of just0.6μm. The duration of the currentless deposition process with such alayer thickness is in the present example about 1 hour.

The result is that a metal solution-diffusion membrane is yieldedcomprising a macroporous ceramic hollow fiber, an intermediate layerwith about 3 to 4 μm thickness as well as an ultra-thin palladium layerwith a thickness of about 0.6 μm applied thereupon.

The hydrogen transport through a palladium membrane is characterized bya solution-diffusion mechanism with the following steps:

a) reversible dissociative chemisorption of H₂ on the membrane surface;

b) volume diffusion of the hydrogen atoms into the metal due to thedriving force of the concentration gradient; and

c) recombination of the hydrogen atoms to molecules on the oppositesurface and desorption.

Usually in thick metal membranes, the hydrogen transport is determinedby the volume diffusion, whereas in the present thin metal membrane, thereaction process of the hydrogen with the metal surface plays thedecisive role.

In comparison to the metallic membrane layer, the intermediate layerdoes not play an additional limiting role for hydrogen transport due toits pore size of approximately 6 nm.

The basic layer buildup of the present solution-diffusion membrane isseen again in FIG. 2. This figure shows a section of the hollow fiber 1with the through-going macropores 4. The intermediate layer 2 is formedon the surface of the hollow fiber 1. This intermediate layer iscomposed of sol particles 5 coated with metal salts 6. Finally the metalmembrane layer 3 is applied onto the intermediate layer.

Such a type metal solution diffusion membrane as produced with theprocess steps of the preferred embodiment described in the precedingpossesses yields excellent long-term stability. The separation factorhydrogen/nitrogen, which is defined by the permeability ratio of purehydrogen to pure nitrogen is more than 1000 in such a type membrane.Besides a high separation surface/volume ratio, the membrane possesseshigh permeability and can, moreover, be produced with little cost,because little expensive metal material is required for the membranelayer.

LIST OF REFERENCE NUMBERS

-   1 macroporous hollow fiber-   2 intermediate layer-   3 metal membrane layer-   4 macropores-   5 sol particles-   6 metal salt

1. A metal solution-diffusion membrane comprising a macroporous baselayer comprising hollow fiber, a metal membrane layer, and anintermediate layer between said macroporous base layer and said metalmembrane layer, wherein said intermediate layer comprises particles of asol, said particles being coated with a salt of a metal of said metalmembrane layer.
 2. A metal solution-diffusion membrane according toclaim 1, wherein said metal membrane layer has a layer thickness in arange between 0.1 and 10 μm.
 3. A metal solution-diffusion membraneaccording to claim 1, wherein said metal membrane layer has a layerthickness in a range between 0.7 and 1 μm.
 4. A metal solution-diffusionmembrane according to claim 1, wherein said intermediate layer has alayer thickness in a range between 1 and 10 μm.
 5. A metalsolution-diffusion membrane according to claim 1, wherein saidintermediate layer has a layer thickness in a range between 2 and 3 μm.6. A metal solution-diffusion membrane according to claim 1, whereinsaid hollow fiber has an outer diameter in a range between 80 and 1500μm, a wall thickness in a range between 10 and 200 μm and an averagepore size of approximately 0.2 μm.
 7. A metal solution-diffusionmembrane according to claim 1, wherein said hollow fiber is formed froma ceramic material.
 8. A metal solution-diffusion membrane according toclaim 1, wherein said hollow fiber is formed from a metal material.
 9. Ametal solution-diffusion membrane according to claim 1, wherein saidmetal membrane layer is formed from palladium or a palladium alloy. 10.A method for producing the metal solution-diffusion membrane accordingto one of claims 1, 2, 3, 4, 5, 6, 7, 8 or 9 comprising: providing atleast one macroporous hollow fiber; applying onto said at least onehollow fiber said intermediate layer, said intermediate layer includingmetallic nuclei for subsequent currentless deposition of the metalmembrane layer; passivating said intermediate layer; and applying saidmetal membrane layer by means of currentless deposition.
 11. A methodaccording to claim 10, wherein said applying of said intermediate layeroccurs by means of applying a Böhmit sol modified with metal complexesand subsequent calcination.
 12. A method according to claim 11, whereinsaid applying of said intermediate layer occurs using a dip coatingprocess, while a vacuum is generated in an interior of said hollowfiber.
 13. A method according to claim 10, wherein said passivatingoccurs by means of hydrogen flowing over said intermediate layer.
 14. Amethod for producing a metal solution-diffusion membrane comprising amacroporous base layer comprising hollow fiber, a metal membrane layer,and an intermediate layer between said macroporous base layer and saidmetal membrane layer, wherein said intermediate layer comprisesparticles of a sol, said particles being coated with a salt of a metalof said metal membrane layer, said method comprising: providing at leastone macroporous hollow fiber; applying onto said at least one hollowfiber said intermediate layer by means of applying a Böhmit sol modifiedwith metal complexes and subsequent calcination, said intermediate layerincluding metallic nuclei for subsequent currentless deposition of themetal membrane layer; passivating said intermediate layer; and applyingsaid metal membrane layer by means of currentless deposition.